Synthesis and Characterization of Structural, Magnetic and Electrical Properties of Ni–Mn–Zn Ferrites
Hosney Ara Begum1,a, Nazia Khatun1,b,*, Suravi Islam1,c, Nurzaman Ara Ahmed1,d , Mohammad Sajjad Hossain1.e, Mohammad Abdul Gafur2,f and Ayesha Siddika2,g
1 Industrial Physics Division, BCSIR Laboratories, Dhaka. Bangladesh.
2 IFRD, BCSIR, Dhaka, Bangladesh.
Received 20April 2017; Revised 13 June 2017; Accepted 25Aug 2017
ABSTRACT
Polycrystalline ferrites (NiXMnZn1−XFe2O4) were synthesized by a conventional ceramic method and sintered at 1100°C for 4 hours. The structure and surface morphology of the samples were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. Frequency‐dependent permeability (μ′), conductivity (σ), quality factor (Q), dielectric loss (tanδ), and dielectric constant (ε′) of the samples were measured by impedance analyzer. DC resistivity and activation energy were also determined. Activation energy was calculated by Arrhenius‐type resistivity plots.
Analysis of the microstructure of the prepared sample showed that the average grain size increased with increasing nickel (Ni) content. Based on the XRD patterns, the prepared samples showed a cubic spinel structure and an average crystalline size of 46 nm.
Keywords: Microstructure, Grain size, Curi temperature (Tc), Dielectric Constant (ε′) and Conductivity (σ).
1. INTRODUCTION
Since the discovery of ferrites, the design and synthesis of ferrites have been the focus of intense fundamental and applied research as such to improve the physical properties of ferrite. Ferrite materials are widely used in electronic devices such as sensors, memory storing apparatus, electromagnetic gadgets, and actuators. Soft magnetic ferrites such as those based on oxides, are the most important [1]. Scholars have reported on the advantages of nano‐crystalline ferrites particularly nano‐sized zinc that interacts with other mixed ferrites [2,3].The magnetic and electric properties [4] of these materials have gained increasing research attention[5]. Mn–Zn ferrites are extensively used in electronics due to high saturation magnetization, magnetic permeability, and dielectric resistivity and low core losses [6,7]. Similar to Mn–Zn ferrites, Ni–
Zn ferrites play a vital role in the technological field.
The combination of the Mn–Zn and Ni–Zn, namely, Ni–Mn–Zn ferrite, may produce good magnetic properties and can be applied in high frequency regions [8].The dielectric properties of Ni–Mn–Zn ferrites should be further studied considering only few works have been conducted on these materials [9]. These ferrites can be synthesized through co‐precipitation, hydrothermal process, solution–gel technique, high‐energy ball milling, and conventional ceramic method [10–15]. In the present work, conventional ceramic method was used for
*Corresponding author: Nazia Khatun, Scientific Officer, Industrial Physics Division, BCSIR Laboratories, Dhaka, Dhaka‐1205, Bangladesh . E‐mail: [email protected]
16
synthesis. The density, porosity, and initial permeability of NiXMnZn1−XFe2O4 were systematically evaluated to investigate the electric and magnetic properties of Ni–Mn–Zn ferrites and extend their application in high frequency regions.
2. MATERIALS AND METHODS
Polycrystalline NiXMnZn1−XFe2O4 was prepared through a simple conventional ceramic method.
Analytical‐grade manganese oxide (MnO2), nickel oxide (NiO), zinc oxide (ZnO), and ferric oxide (Fe2O3) were used as raw materials. Stoichiometric amounts of raw materials were mixed in a soggy medium to facilitate homogeneous mixing. Aluminum balls were used to prevent iron contamination, and ethanol was added to form homogeneous slurry. The samples were pulverized through a planetary ball mill for 8 hours. The weight ratio of the ball to powder was 10:1. The milled samples were calcined in a furnace at 700°C for 3 hours in the presence of air.
The dried powder was ball milled for 2 hours. The ground ferrite powder was plasticized with an aqueous solution of 4% polyvinyl alcohol (PVA) as binder. The resulting materials were pressed into pellets and toroidal shape by applying 3±0.5 MPa for 1 minute. The samples were then sintered at 1100°C for 4 hours.
3. RESULTS AND DISCUSSION
NiXMnZn1−XFe2O4 ferrites were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The XRD analysis showed a single phase patterns indicating that the sample is ferrite with a cubic spinel structure. All XRD peak in Figure 1 is at (220), (311), (222), (400), (422), (511), and (400) planes. Generally, the intensity of the XRD peak changes when the structure is altered. Such changes also depend on the atomic position, type and quantity in the cell. In addition, grain size, grain surface relaxation and temperature also influence the variations in the peak intensity.
25 30 35 40 45 50 55 60 65 70
x=0.2 x=0.4 x=0.6 x=0.8
(440)
(511)
(422)
(400)
(222)
(220)
Intensity
2 Theta
(311)
Figure 1. XRD pattern of NiXMnZn1−XFe2O4 (where x= 0.2, 0.4, 0.6, 0.8)
By using the Debye Scherrer equation in Eq. (1), the average crystallite size can be determined from the (311) diffraction peak, which exhibited the highest intensity.
. (1)
17 where is the wavelength of X‐ray (1.5406Å), is the full width at half maximum (in radians), and θ is the Bragg’s angle. The lattice parameter of the cubic samples was calculated from d‐
spacing corresponding to the strongest peak (311) according to the following relation:
(2) where h, k, and l represent the Miller indices of the crystal planes.
Table 1 shows the lattice constant, X‐ray density, bulk density, and porosity of the samples. The lattice parameters of all samples were plotted against Ni content as shown in Figure 2. Results indicate that the lattice constant decreased with increasing Ni content. The lattice parameter was then used to calculate X‐ray density.
Figure 2. Variation in the lattice constant with Ni content
The relation between X‐ray density, and lattice constant, a is as follows:
(3)
where N is the Avogadro’s number (6.023×1023 m−1), and M is the molecular weight. The actual density of the ferrite samples was determined using the Archimedes’ principle. The porosity (%P) of the samples was then calculated using the following equation:
P = (1 − / × 100 (4) where is the actual density of the sample determined using the formula in Eq. (5):
= (5) where m is the sample weight, and v is the sample volume.
8.395 8.4 8.405 8.41 8.415 8.42 8.425 8.43 8.435 8.44 8.445
0 0.2 0.4 0.6 0.8 1
lattice constant a in Å
Ni content, x
18
Table 1 Lattice constant, X‐ray density, bulk density, and porosity of the samples Ni content lattice constant a
in Å X‐ray density,
gm/cc Bulk density,
gm/cc Porosity, P (%)
X=0.2 8.4336 6.1928 4.9554 19.9798 X=0.4 8.4328 6.1649 4.9554 18.0408 X=0.6 8.4197 6.1639 4.9699 19.3708 X=0.8 8.3989 6.1798 4.7291 23.4748
The microstructure of NiXMnZn1−XFe2O4 ferrites was examined through SEM analysis and the images was provided in Figure 3. The microstructure of the synthesized sample shows normal grain growth, uniform and homogeneous distribution of Ni‐Mn‐Zn ferrites.
Figure 3. SEM images of NiXMnZn1−XFe2O4, where Sample 1(S1) X= 0.2; Sample 2 (S2) X= 0.4; Sample 3 (S3) X= 0.6; and Sample 4 (S4) X= 0.8
The average grain size was calculated from the microstructure of the surface in the SEM image.
The results are as follows: S1 = 0. 656 μm, S2 = 1.083 μm, S3 = 1.179 μm, and S4 = 1.0180 μm.
Grain diameter was determined eight or ten times at random, and the average value was obtained. The change in the grain size could be due to the increase in the Ni content, that is, the grain size increased with increasing Ni content. Moreover, the ionic radii of Ni increased when the particle size was increased. The grain size increased when Ni content was set as X=0.6 and decreased thereafter. Table 2 and Figure 4 show the changes in grain size with increasing Ni content.
S1 S2
S3 S4
19 Table 2 Grain size variation in NiXMnZn1−XFe2O4 samples
Composition Grain size ( µm )
Ni0.2MnZn0.8Fe2O4 0.656375
Ni0.4MnZn0.6Fe2O4 1.083
Ni0.6MnZn0.4Fe2O4 1.179
Ni0.8MnZn0.2Fe2O4 1.0180
Figure 4. Concentration of Ni versus grain size
Based on the Arrhenius’ relation, activation energy was calculated using the formula:
ρ =ρ0exp(Ea/kT) (6)
where Ea is the activation energy, k is the Boltzmann constant, and ρ is the resistivity at absolute temperature, T. The results are shown in Table 3.
Table 3 Variations in the Curie temperature and activation energy of NiXMnZn1−XFe2O4 samples No. of
sample Composition Resistivity Ω‐cm in
room temperature Curi
temperature (K) Activation Energy Ea in eV
1 Ni0.2MnZn0.8Fe2O4 8.456 X 106 383 0.5289
2 Ni0.4MnZn0.6Fe2O4 9.163 X106 413 0.4128
3 Ni0.6MnZn0.4Fe2O4 1.5295X107 498 0.3986
4 Ni0.8MnZn0.2Fe2O4 1.5691X107 458 0.3954
Figure 5 shows the plot of the Curie temperature (Tc) of the sample versus 1000/T. The Curie temperature (Tc) is an essential property of ferromagnetic materials. The lnρ versus 1000/T curves of all the samples were of the same characteristics up to a definite temperature. As shown in Table 3 and figure 6(a), the value of Tc increased from sample S1 (x=0. 2) to S3 (x=0. 6) but decreased in S4 (x=0.8). Figure 6(b) shows the activation energy slightly decreased with Ni content, and this result is similar to the report by Sattar et al. [16]. Moreover, the resistivity and
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
0.1 0.3 0.5 0.7 0.9
Grain size in μm
Concentration of Ni content
20
Tc increased with Ni content. The increase in the Tc could be attributed to the decrease in the A–
B interaction. As the Ni content increases, the relative number of Fe2+ ions on the A site increases, thereby reducing the A–B interaction [17].
5 10 15 20 25 30
1 2 3 4 5 6 7 8
Resistivity cm
(1000/T K)
X=0.2 X=0.4 X=0. 6 X=0.8
1000/T K Vs Resistivity
Figure 5. 1000/T versus resistivity of NiXMnZn1−XFe2O4.
Figure 6. (a) Concentration versus Curie temperature and (b) Concentration versus activation energy
The frequency dependence of dielectric constant (έ) for all the samples was studied at room temperature. Figure 7 (a) shows the variation in dielectric constant with frequency. The value of έ decreased continuously with increasing frequency. The frequency dependence of the samples (Ni–Mn–Zn ferrites) showed a normal dielectric behavior, similar to that described by Radha and Ravinder [18]. This reduction occurs because beyond a certain frequency of the externally applied electric field, the electronic exchange between Fe2+ ↔ Fe+3 ions cannot follow the alternating field. This behavior is in good agreement with the result of our previous work [19].
According to Refs. [20] and [21], the conduction mechanism of ferrites is strongly correlated with their dielectric behavior. The variation in dielectric loss with frequency is shown in Figure 7 (b). The dielectric loss decreased when the frequency of the applied AC electric field is lower than the hopping frequency of electrons between Fe+2 and Fe+3 ions at neighboring octahedral sites where the electrons follow the field leading to maximum loss. At high frequencies of the applied electric field, the hopping frequency of the electron exchange cannot follow the applied field beyond certain critical frequency, and the loss is at the minimum. The maximum tanδ of
360 410 460 510
0 0.2 0.4 0.6 0.8 1
Curie temperature
Concentration of Ni content Concentration of Ni content Vs
Curie temparature
0.35 0.4 0.45 0.5 0.55
0 0.5 1
Activation energy eV
Concentration of Ni content Cocentration of Ni content Vs
activation energy in eV
21 NixMnZn1−xFe2O4 was observed at 75 KHz, and the minimum dielectric loss tangent value was detected at 10 MHz frequency. The tanδ decreased gradually with increasing frequency.
10 100 1000 10000 100000 1000000
0 200 400 600 800 1000 1200 1400 1600 1800
Dielelctric constant/
Frequency in Hz
x=0.2 x=0.4 x=0.6 x=0.8 Frequency Vs Dielectric Constant
200000 400000 600000 800000 1000000
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
Frequency in Hz
X=0.2 X=0.4 X=0.6 X=0.8 Frequency Vs Dielectric loss
Dielectric loss, tan
Figure 7 (a). Frequency versus dielectric constant Figure 7 (b): Frequency versus dielectric loss
The variations in AC resistivity and conductivity are shown in Figure 8 (a) and 8 (b) respectively. The AC resistivity and conductivity of Ni–Mn‐Zn ferrites were collected frequency range of 1 MHz to 10 MHz at room temperature by using an impedance analyzer. The AC resistivity decreased with increasing frequency. By contrast, the AC conductivity (σ) increased with increasing frequency for all the samples. Thus, the resistivity and conductivity showed opposite characteristics in response to the frequency. Koops theory and Maxwell–Wanger theory [22–24] explained that the AC conductivity increases with increasing frequency, and this trend is in agreement with experimental results in this study. The frequency dependence of resistivity can be explained by the Verwey’s hopping mechanism [25]. At low frequencies, the grain boundaries are active and the hopping activities of ferrous and ferric (Fe2+& Fe+3) ions are low. As the frequency of the applied field increases, the conductive grains become more active, thereby promoting the hopping mechanism between Fe2+ and Fe+3 ions and increasing the hopping conduction. The conductivity gradually increased with frequency.
0 200000 400000 600000 800000 1000000
0 1000000 2000000 3000000 4000000 5000000 6000000 7000000 8000000
Resistivity (-cm)
Frequency in Hz
X=0.2 X=0.4 X=0.6 X=0.8
Frequency Vs Resistivity
0 200000 400000 600000 800000 1000000
0.0000005 0.0000010 0.0000015 0.0000020 0.0000025
Conductivity(S/m)
Frequency in Hz
X=0.2 X=0.4 X=0.6 X=0.8 Frequency Vs Conductivity
Figure 8 (a). Frequency versus resistivity Figure 8 (b).Frequency versus conductivity Figure 9 shows the plot of initial permeability as a function of frequency for the samples sintered at 1100°C. The value of μi increased with increasing Ni content because of increased grain size and enhanced magnetization [26]. The initial permeability of the sample was influenced by the frequency. Hence, high initial permeability can be obtained by increasing the Ni concentration, but the frequency stability decreases. Moreover, the permeability decreased rapidly in low frequency regions and becomes almost constant. The permeability of ferrites is associated with two types of mechanisms, namely, spin rotation magnetization inside the domains and domain wall motion [27–28]. Figure 10 shows the plot of frequency versus Q‐
22
factor for Ni–Mn–Zn ferrites. The Q‐factor increased gradually with increasing frequency.
Ferrites exhibit high Q‐factor and DC resistivity of the order of 106 Ω‐cm and thus are very useful for transformer core applications up to few MHz frequencies.
10000 100000 1000000 1E7 1E8
50 100 150 200 250 300
Initial permeabilityi
Frequency in Hz
X=0.2 X=0.4 X=0.6 X=0.8 Frequency Vs Initial Permeability
Figure 9. Frequency versus initial permeability
100 1000 10000 100000 1000000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Quality factor, Q
Frequency in Hz
X=0.2 X=0.4 X=0.6 X=0.8 Frequency Vs Quality Factor
Figure 10.Frequency versus quality factor
4. CONCLUSIONS
In this study, NiXMnZn1–XFe2O4 ferrites which play an important role in enhancing magnetic, electrical, and dielectric properties was successfully synthesized. Both the bulk and X‐ray densities of the samples decreased with increasing Ni content. The XRD pattern confirmed the single‐phase cubic spinel structure. Magnetic properties, such as initial permeability, were also assessed as a function of frequency. The real part of permeability decreased gradually with increasing Ni content. The decrease in the dielectric constant with increasing frequency could be due to the electronic exchange between Fe2+ and Fe3+ ions in octahedral sites, and this exchange does not follow the frequency of the applied AC field. When increasing Ni content, the Curie temperature and DC resistivity increased but the activation energy decreased. These
23 experimental results provide a basis for the promising high‐frequency applications of ferrite materials.
ACKNOWLEDGEMENT
We are grateful to BCSIR for granting us the permit to carry out this research work. We would like to express our gratitude to Dr. Md. Toffazal Hossain, a scientist from BCSIR, for his help and cooperation. Finally, we would like to thank all the scientists and staff of Industrial Physics Division, BCSIR Laboratories, Dhaka for their assistance.
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